According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view, the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field of the RF pulse extends perpendicular to the z-axis, so that the magnetization performs a precession about the z-axis. This motion of the magnetization describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle 90°). The RF pulse is radiated toward the body of the patient via a RF coil arrangement of the MR device. The RF coil arrangement typically surrounds the examination volume in which the body of the patient is placed.
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF antennas or coils which are arranged and oriented within the examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving RF antennas or coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation or by other per se known reconstruction techniques.
As described more detailed herein below, thermal energy deposition is increasingly used in medicine as a means of necrosing diseased tissues. The present invention is disclosed in the following in the context of therapeutic thermal treatment by high intensity RF irradiation. A thermal treatment applicator comprising an array of RF antennae is used for generating a RF electromagnetic field within a target zone of the body tissue to be treated. The thermal treatment applicator used in therapy is typically located outside the body over the region to be treated.
A therapeutic system comprising a thermal treatment applicator is generally known, e.g., from US 2010/0036369 A1.
In RF hyperthermia therapy the tissue of interest is irradiated with high intensity RF electromagnetic radiation which is absorbed and converted into heat, raising the temperature of the tissue. As the temperature rises above 55° C., coagulative necrosis of the tissue occurs resulting in cell death.
RF hyperthermia therapy can be advantageously combined with MR imaging, thereby enabling imaging guided local therapy. MR thermometry, based on the proton resonance frequency shift (PRFS) in water, is presently considered the ‘gold standard’ for the non-invasive monitoring of thermal therapies. Temperature-induced changes in the proton resonance frequency are estimated by measuring changes in phase of the acquired MR signal by means of appropriate and per se known MR imaging sequences.
In conventional thermal treatment applicators the mutual coupling of the individual RF antennae is a limiting factor for the design of optimized RF electromagnetic field characteristics. Further disadvantages of known applicator designs are significant RF power loss in feeding cables, via which the RF energy is supplied from the RF power amplifiers to the RF antennae, as well as the use of inefficient RF power amplifiers. Power efficiency of the RF power amplifiers results not only in wasted power but also in a large heat production. The heat load on the RF electronics makes the design more expensive and bulky, because provision has to be made for an appropriate cooling system, and/or negatively impacts reliability of the system.
From the foregoing it is readily appreciated that there is a need for an improved MR imaging guided therapy technique.